The objective of this proposal is to characterize the structure, diversity and expression of recently defined surface membrane proteins of Mycoplasma fermentans, a small wall-less procaryote recently identified as a potential opportunistic agent directly associated with the pathobiology of human acquired immunodeficiency syndrome (AIDS), and with fatal non-AIDS disease in humans and primates. These antigens are prominent, integral membrane proteins modified by lipid. They have a potentially major role as mediators of host cell interactions, modulators of the immune response and as variant structures whose differential expression may allow avoidance of immune recognition. Moreover, they are important as potential targets for serological analysis of infectious epidemiology and disease association. Several of these structurally diverse surface lipoproteins recently defined by monoclonal antibodies will be analyzed. These proteins include strain variant antigens, and all undergo high-frequency phase variation in vitro. This feature may reflect an important diversity-generating system providing these organisms with versatile adaptive capability that could enhance their potential as infectious agents and pathogens. It is proposed to (i) sequence currently identified and additional cloned genes encoding these antigens, (ii) overexpress recombinant proteins for their assessment as targets in serologic assays, and (iii) compare structural and regulatory features of these genes in isogenic, clonal lineages representing phase transitions in expression, in order to determine the molecular basis for. antigenic and phase variation. Standard methods will be used to identify, clone, sequence, and mutationally alter and express genes; to immunologically monitor antigenic recombinant proteins; and to monitor gene expression in vitro by analysis of transcripts.